Ti alloys have high specific strength and superior corrosion resistance and are widely used in various fields such as in the field of aviation and the field of chemical plants. In this forming, use of a superplasticity (hereinafter referred to “superplastic forming”) is particulary effective. The superplasticity is also applied in joining processing, and in particular, the combined processing of superplastic forming and diffusion bonding (SPF/DB) is practically applied in the field of aviation.
In conventional Ti-6Al-4V alloys, in order to cause the superplasticity, forming is performed at a high temperature of about 800 to 950° C. and in a condition of plastic deformation at a low strain ratio of 1×10−4 to 10−3/sec. However, since the forming is performed at high temperature at a low deformation rate, the production efficiency is low and deterioration of mechanical properties easily occurs due to oxidization of the material and coarsening of crystal grains in the superplastic forming. Furthermore, there is a problem in that the service life of a die is short since forming is performed at a high temperature. The superplastic forming of Ti-6Al-4V alloys is an attractive process which enables near-net shape forming. However, the superplastic forming with conventional Ti-6Al-4V alloys has various problems as mentioned above, and the applicable range is limited. Therefore, it is strongly desired for Ti alloys to cause the superplasticity at a low temperature at a high deforming rate.
Until now, it has been reported in “METALLURGICAL TRANSACTIONS” (J. A. Wert and N. E. Paton, 1983, A14, p. 2535-2544) that the superplastic forming temperature can be reduced by alloy design in which the ratios of the amounts of an α phase and a β phase are adjusted. In JP3-274238, a Ti-4.5Al-3V-2Mo-2Fe alloy in which the superplastic forming temperature can be at least 100° C. lower than that of Ti-6Al-4V alloy by suitable alloy design, was developed. On the other hand, as a method for causing the superplasticity at a low temperature at a high deforming rate, refinement of crystal grains may be exemplified. For example, it has been reported that an ultrafine structure of average grain size of 0.5 μm or less is formed by utilizing severe plastic deformation processing in a Ti-6Al-4V alloy, can lower the superplastic forming temperature by 150 to 250° C. than the conventional structure, and can cause the superplasticity at high forming rates (strain rates) of 1×10−3 to 10−2/sec (“Journal of Materials Processing Technology” (G. A. Salishchev et al., 2001, 116, pp. 265-268), “Materials Science and Engineering” (R. S. Mishra et al., 2001, A298, pp. 44-50), “JOURNAL OF MATERIALS SCIENCE” (G. A. Salishchev, O. R. Valiakhmetov, R. M. Galley, 1993, 28, pp. 2898-2902), “Materials Science Forum” (G. A. Salishchev, O. R. Galeyev, S. P. Malysheva, O. R. Valiakhmetov in ICSAM′97 (Ed. A. H. ChokShi), 1997, 243-245, pp. 585-591), “Materials Science and Engineering” (Y. G. Ko et Al., 2005, A 410-411, pp. 156-159), “Formation of fine grain structure following to super severe deformation” (Nobuhiro TSUJI, Iron and Steel, 2008, 94, pp. 582-589)). Lowering temperature and increasing deforming rate in the superplastic forming results not only in efficient production, but also prevents oxidization of material, inhibits deterioration of mechanical properties, prolongs the service life of dies, and decreases the total forming cost.
However, the severe deformation process is a method in which an amount of strain of 4 to 5 is introduced in the material and performed by methods such as ECAP (Equal Channel Angular Pressing) method, HPT (High Pressure Torsion) method, MM (Mechanical Milling) method, ARB (Accumulative Roll-Bonding) method, multi-axis forging method, and high speed shot peening method. Such severe deforming processes require introduction of a large amount of strain, and this is not suitable for production of large material for forming and mass production. For example, a Ti-6Al-4V alloy which was formed (strain amount, ε=8) by the ECAP method (reported in “Materials Science and Engineering”) and a Ti-6Al-4V alloy which was formed (strain amount, c=7) by the HPT method (reported in “Scripta MATERIALIA” (A. V. Sergueeva et al., 2000, 43, pp. 819-824) exhibit superplasticity at temperatures of 650° C. and 700° C. However, the strain amount introduced in the materials is identical to deformation in which a ingot 450 to 1000 mm thick is rolled to 1 mm thick at one time rapidly, whereby, this is not realistic in a production process of a plate using a simple rolling process. It should be noted that most of the practical materials for superplastic forming are provided as plate materials, primarily as airplane structural parts. Therefore, it has been strongly desired to provide techniques for practical superplastic forming processes in α+β type Ti alloys, which are easy to obtain by relatively low-cost and are widely used.
Refinement of crystal grains of Ti alloys contributes not only to improvement of superplasticity, but also yields great improvement in mechanical properties such as strength and fatigue properties. Therefore, refinement of crystal grains is effective as a method for improving several material properties together.